30 IOWA ACADEMY OF SCIENCES. 



interest for the reason that it has long been the only altered formation 

 known within the limits of Iowa. The apparent metamorphic characters 

 of the qnartzite beds is all the more remarkable from the fact that the rocks 

 of the State are so horizontal in their position, so undisturbed by mountain 

 making forces, and so unchanged in lithological characters that it is com- 

 monly supposed that all the strata of the State are essentially the same as 

 when deposited in the quiet waters of the great interior sea which once 

 occupied the heart of the American continent. 



Although so thoroughly ci-ystalliue and so closely resembling quartzitic 

 rocks altered from sand beds through regional or contact metamorphism no 

 massive crystallines have been mentioned in connection with the Sioux 

 quartzite until quite lately when a large mass of diabase was discovered in 

 the midst of the quartzite of southeastern Dakota. Still more recent borings 

 in northwestern Iowa have revealed no great distance below the surface 

 other rocks of undoubted eruptive origin among which may be mentioned 

 quartz-porphyry. 



The Sioux quartzite formation has received considerable attention from 

 time to time, but for the most part the observations have been somewhat 

 cursory; incidental to other examinations rather than special examinations. 



Irving's description of the lithological featui'es of this formation essen- 

 tially agrees with observations made during the past few mouths. It is as 

 follows: 



"Loose sandstone to the hardest and most complete vitreous quartzite, 

 the prevalent phase being a distinctly quartzitic one. The loosest and most 

 completely indurated portions are arranged in the most iri'egular relations 

 to one another. Occasionally they will be iuterstratified. At times the 

 exposed parts will be completely vitrified, while below artificial openings will 

 display an entirely loose sandstone, suggesting an induration of the exposed 

 portions by weathering. In other cases, however, exactly the I'everse of 

 this will be met with, while very often the more and less indurated phases 

 pass into each other latei'ally by I'apid graduations, the two phases traversing 

 the layers and dovetailing iato each other in the most irregular manner. 

 The prevalent color of the formation is red, but the loosest varieties are 

 often very pale coloi'ed, while the most vitreous kinds frequently present a 

 very dark purple hue. In western Minnesota and again in certain points of 

 Dakota, there is associated with the quartzite the fine clayey rocks known 

 as pipestone, or catlinite. Intermediate between this pipestone and the 

 purely silicious quartzite are clayey sandstones and quartzites, often of a 

 blotched appearance, and not a little resembling externally certain of the 

 Keweenavvan sandstones of Lake Superior. So far as the microscope studies 

 have gone these rocks are in the main mixtures of red clay and quartz. Con- 

 glomeritic phases of the quartzite are met with at a number of points, but 

 no other rocks but those already mentioned have been recognized in this 

 great formation." 



Thin sections under the micx'oseope show that the great sandstone beds 

 have become consolidated and rendered quartzitic through the secondary 

 enlargement of the sand grains, by additions of silica, the added parts being 

 oriented optically with the internal grains they surround. In South Dakota a 

 few miles northeast of Carson station on the Sioux City & Northern Rail- 

 road, there is exposed in the i-ailroad cut some sections which show an alter- 

 nation of thin layers of the hardest quartzite and soft incoherent sands. In 



